Vanishing Atomic Particles Suggest Need for a 'New Physics'

By JOHN NOBLE WILFORD

Published: December 29, 1991

After two years of trying to count neutrinos coming from the Sun, Russian and American scientists are becoming convinced that something strange is happening to these elusive subatomic particles on their way out of the Sun. Nearly all of them vanish, perhaps escaping detection by changing character.

The scientists reported this month that a new, more rigorous analysis of their observations had confirmed preliminary findings that virtually no solar neutrinos can be detected reaching Earth, even though the nuclear-fusion processes occurring at the Sun's core are presumably producing a profusion of these particles. Neutrinos have no electric charge and little or no mass.

Highly sensitive detectors on Earth should have identified the traces of at least 17 neutrino impacts over a six-month period; only three were found. These results, reported in a recent issue of Physical Review Letters, confirmed earlier findings by the Russian-American research team. They strengthened the view among scientists that a "new physics" will have to be developed to account for the missing solar neutrinos, one of the most puzzling and provocative mysteries in the physical sciences.

One potentially important implication of the new physics would be that neutrinos are not devoid of mass, as previously supposed, and so could account for much of the unseen mass, or dark matter, that cosmologists presume must exist in the universe. This hidden matter is considered crucial to understanding the structure and evolution of the universe. Other recent research tends to support the concept of neutrinos having some slight mass. Experiment in a Mine

Billions of solar neutrinos, invisible and without electric charge, pass through every square inch of the Earth every second. But the particles have so little affinity for normal matter that they pass through unhindered and undetected, except for a rare collision with other subatomic particles that leave a trace. Such a collision, for example, would change an atom of the element gallium into a form of the element germanium.

In the Soviet-American Gallium Experiment, called SAGE by the participants, 30 tons of the liquid metal gallium deep in a mine in the Caucasus Mountains serves as the detector. Measuring the germanium in the containers after at time tells the scientists how many neutrinos collided with the gallium.

American scientists on the project said the new study all but ruled out the possibility that the deficit in detected neutrinos could be a result of flaws in the experiment itself.

"We now have more data, we've done more analysis and we've been able to push down the limits of detection," said Dr. Thomas Bowles, a nuclear physicist at the Los Alamos National Laboratory who is working on the experiment. "We have done several systematic checks of the experiment and everything works correctly."

Dr. Bowles and others at Los Alamos suggested two possible explanations for the findings. Either scientists do not understand the Sun's internal operation as well as they think they do, or neutrinos change character after being generated by solar fusion, a conclusion that would require major revisions of the standard model for the behavior of particles. Opportunities for Interactions

Dr. John N. Bahcall, a specialist in solar neutrinos at the Institute for Advanced Study at Princeton, N.J., said that he found the new evidence "very convincing" and that the most likely explanation involved "some new physics, not new solar astronomy." He said this was the "overwhelming consensus" of physicists at a recent workshop on neutrinos.

One aspect of the new physics not only would be based on the assumption that neutrinos have some mass, scientists said, but that the three types of neutrinos -- the electron, tau and muon neutrinos -- are capable of some interactions. Only the electron solar neutrinos are detectable with current instruments; the other two are observable only in nuclear accelerators.

According to a theory developed by Russian and American scientists six years ago, if electron neutrinos have a tiny mass, their interaction with ordinary matter could cause them to oscillate. Inside the Sun, dense with matter, there would be ample opportunities for such interactions. And in many cases, the oscillations might transform an electron neutrino into a muon neutrino and thus render it unobservable with present detectors on Earth.

This is called the M. S. W. effect, for the three scientists, Dr. Stanislav P. Mikheyev and Dr. Alexi Smirnov of the Soviet Academy of Sciences, who originated the theory, and Dr. Lincoln Wolfenstein of Carnegie-Mellon University, who had some of the original ideas.

Making calculations based on this neutrino oscillation theory, Dr. Bahcall and Dr. Hans Bethe, a Nobel Prize-winning physicist at Cornell University, recently predicted how many neutrinos the SAGE detector should see, and their prediction agreed with the reported results.

"The M. S. W. solution is likely to be the explanation for the whole solar neutrino problem," said Dr. Peter Rosen, dean of science at the University of Texas at Arlington, who is a theoretical physicist. "Neutrinos will turn out to have a mass, and there will be mixing between their different states."

Dr. Bowles said the gallium detector's results also agreed with earlier findings by neutrino detectors in Japan and South Dakota. "Several different experiments are consistent with one another," he said, "and the only apparent explanation is that neutrinos oscillate."

Within the next year or so, physicists said, new findings by other detectors in Italy and Canada could resolve the solar neutrino mystery.